Ceramic foam and process for producing the same

ABSTRACT

A ceramic foam consisting of two layers in and out, in which a highly pure ceramic shell covers up the surface layer of the fly ash, is provided along with a process for producing such a ceramic foam. The process for producing a ceramic foam  10  includes the steps of: coating the surface of raw fly ash material containing impurities  12  with alumina slurry; granulating the coated fly ash; and sintering the granulated fly ash in a manner that the granulated fly ash is heated in a furnace with raising temperature, then kept at a predetermined temperature for several hours and then, slowly cooled down to room temperature. The impurities are sealed inside the fly ash  11  not to transfer to the outside so that the surface of the fly ash is covered up with a ceramic shell  13  to form the ceramic foam  10  having two layers in and out.

This application is a division of Ser. No. 09/912,339 filed Jul. 26,2001, now U.S. Pat. No. 6,610,359.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a ceramic foam, which is producedstarting from fly ash discharged from a thermal power plant and the likeor pulp sludge discharged from a paper mill as main raw materialsthereof and is applied to a light aggregate for concrete, and processfor producing the same.

(2) Description of the Related Art

In a thermal power plant in which coal is used for boiler fuel, a largequantity of fly ash is discharged as the industrial waste. Recently,such a fly ash has been effectively used as a ceramic aggregate forconcrete of building materials. The fly ash is further expected to beput to practical use as reclaimed resources of a large quantity, whichis free from care of being exhausted for the time being, as well as thepulp sludge.

For example, Japanese Patent Application Laid-Open No. H10-297977discloses a technique that powder such as aluminum oxide (alumina)powder is bound to the surface of fly ash followed by sintering so as toproduce a hollow ceramic foam.

FIGS. 3A and 3B are a cross sectional view schematically illustrating aceramic foam 1 before and after sintering, respectively, which isproduced by such a conventional process as disclosed in the JapanesePatent Application Laid-Open No. H10-297977. As shown in FIG. 3A,ceramic powder 4 such as Al₂O₃, MgO and SiO₂ powder is coated on thesurface layer of fly ash 2 containing impurities 3 such as Fe₂O₃followed by granulation so as to form a ceramic powder layer. In asintering step after the granulation, the granulated fly ash is sinteredat 1100-1300° C., thereby a ceramic foam 1 having low density and highmechanical strength is produced.

The ceramic foam 1 thus produced according to the conventional processhas the following disadvantage.

That is, when the fly ash 2 is coated with the ceramic powder 4 on thesurface layer thereof, then granulated and then, sintered, as shown inFIG. 3B, the impurities 3 (such as Fe₂O₃) contained in the fly ash 2transfer to the ceramic powder 4 and fuses together with the ceramicpowder 4. As a result, a desired ceramic foam, i.e. a hollow particleconsisting of two layers in and out, in which a highly pure ceramicshell covers up the surface layer of the fly ash, cannot be produced.

SUMMARY OF THE INVENTION

It is therefore an objective of the present invention to solve the aboveproblem and to provide a high quality ceramic foam consisting of twolayers in and out, in which a highly pure ceramic shell covers up thesurface layer of the fly ash, and a process for producing such a ceramicfoam.

In order to attain the above objective, the present invention is toprovide a ceramic foam having two layers in and out consisting of: aninner layer of hollow fly ash part produced by a process, in which rawfly ash material containing impurities is coated with alumina slurry onthe surface thereof, then granulated and then, sintered; and an outerlayer of a ceramic shell part produced by said process covering up thesurface of the hollow fly ash part.

According to the constitution described above, a ceramic foam as a highquality hollow particle consisting of two layers in and out, in which aceramic shell covers up the surface layer of the hollow fly ash, can beprovided.

In order to attain the above objective, the present invention is also toprovide a process for producing a ceramic foam comprising the steps of:coating the surface of raw fly ash material containing impurities withalumina slurry; granulating the coated fly ash; and sintering thegranulated fly ash in a manner that the granulated fly ash is heated ina furnace with raising temperature, then kept at a predeterminedtemperature for several hours and then, slowly cooled down to roomtemperature, whereby the impurities are sealed inside the fly ash not totransfer to the outside so that the surface of the fly ash is covered upwith a ceramic shell to form the ceramic foam having two layers in andout.

According to the process described above, the impurities contained inthe fly ash are prevented from transferring to the alumina slurry, whichis coated on the outer surface of the fly ash, and sealed inside the flyash. As a result, a ceramic foam as a hollow particle consisting of twolayers in and out, in which a ceramic shell uniformly covers up thesurface of the hollow fly ash, can be provided.

Preferably, in the step of sintering, a rate of the heating is 5°C./minute, the predetermined temperature ranges from 1350° C. to 1460°C., the several hours is three hours, and a rate of the slow cooling is5° C./minute.

According to the process described above, the impurities contained inthe fly ash are prevented from transferring to the alumina slurry andfrom fusing together with the alumina slurry.

Preferably, air is introduced into the furnace with the amount of 100mL/minute per amount of the granulated fly ash 2.0 g in the step ofsintering.

According to the introduction of air with the appropriate amount asdescribed above, a process in which the alumina slurry forms the ceramicshell can be effectively promoted.

Preferably, the alumina slurry has a composition consisting of 65-70 wt% of aluminum oxide, 2-3 wt % of ammonium polycarboxylate as adispersing agent, 2-3 wt % of acrylic emulsion as a binder, and 25-30 wt% of ion exchanger.

By setting up the composition of the alumina slurry as described above,the ammonium polycarboxylate as a dispersing agent prevents the aluminaslurry particles from agglomerateing with each other, the acrylicemulsion as a binder promotes the sintering of the alumina powder, andthe ion exchanger acts as a solvent of the alumina slurry, thereby thedesired ceramic foam as a hollow particle in accordance with theparticle design.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view schematically illustrating a ceramicfoam produced by a process according to a preferred embodiment of thepresent invention;

FIG. 2 is a graph illustrating a relation between sintering temperatureand sintering time in a preferred embodiment of the present invention;

FIG. 3A is a cross sectional view schematically illustrating a ceramicfoam before sintering produced by a conventional process; and

FIG. 3B is a cross sectional view schematically illustrating a ceramicfoam after sintering produced by a conventional process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, a ceramic foam and a process for producing the sameaccording to the preferred embodiments of the present invention will beexplained with reference to the attached drawings.

FIG. 1 is a cross sectional view schematically illustrating a ceramicfoam 10 produced by a process according to a preferred embodiment of thepresent invention. Fly ash 11, which may contain a hollow space 14, isprepared as the main raw material and alumina slurry in which aluminumoxide (alumina) is subjected to slurrying is also prepared. To prepareand use the alumina slurry for producing a ceramic foam in the presentinvention is different from that alumina powder is used in aconventional technique. The fly ash II is coated with thus preparedalumina slurry on the surface thereof and then granulated. In order touniformly coat the alumina slurry on the surface of the fly ash II, thealumina slurry has the following composition. A dispersing agent such asammonium polycarboxylate, a binder such as acrylic emulsion, and ionexchanger such as ion exchange water and organic solvent are added toalumina powder, thereby the alumina slurry based on a particle design isprepared.

A dispersing agent such as ammonium polycarboxylate, a binder such asacrylic emulsion, and ion exchanger such as ion exchange water andorganic solvent are added to alumina powder, thereby the alumina slurrybased on a particle design is prepared.

Two to three wt %, preferably 3 wt %, of ammonium polycarboxylate as adispersing agent is added to 65-70 wt %, preferably 65 wt %, of aluminumoxide. The addition of the ammonium polycarboxylate improves thedispersibility of the aluminum oxide particles in the alumina slurry.Then, 2 to 3 wt %, preferably 2 wt %, of acrylic emulsion as a binder isadded thereto. The addition of the acrylic emulsion improve thesintering property of the aluminum oxide. Further, 25-30 wt %,preferably 30 wt % of ion exchange water is added thereto. The additionof the ion exchange water allows the dispersing agent and the binder tospread over every surface of the aluminum oxide powder, therebypromoting the formation of the alumina layer on the fly ash. The weightratio of the fly ash to the aluminum oxide is set up, for example, 2 to3.

The fly ash is coated with the alumina slurry described above, thengranulated and then, dried at about 25° C. Then, the dried granulatedparticles are sintered in an electric furnace. FIG. 2 is a graphillustrating a relation between the sintering temperature and sinteringtime. That is, the sintering condition is as follows.

Normally, the dried granulated particles are heated with a low heatingrate of 5° C./minute from room temperature (15 to 35° C.) to atemperature ranging from 1350° C.-1460° C.

When the temperature reaches, for example, 1450° C., the granulatedparticles is kept heated at 1450° C. for about three hours.

Thereafter, the granulated particles are cooled down to room temperaturewith a low cooling rate of 5° C./minute.

Thus, the dried granulated particles are slowly heated, then kept heatedfor a predetermined period of time and then, slowly cooled down, therebythe alumina slurry is prevented from fusing together with the fly ash11. Therefore, in contrast to the conventional process, the impurities12 contained in the fly ash 11 are prevented from transferring to thealumina slurry and from fusing together with the alumina slurry in theprocess according to the present invention. That is, a highly pureceramic shell 13 can be formed on the surface of the fly ash 11 and ahigh quality ceramic foam 10 of a hollow particle having two layers inand out can be produced.

Preferably, air is introduced into the furnace, for example, with theamount of 100 mL/minute per amount of the granulated fly ash 2.0 g whilethe dried granulated particles is sintered in the furnace, therebygaseous impurities are prevented from adhereing to the surface of thegranulated particle during the sintering and the formation of theceramic shell 13 can be effectively promoted.

The aforementioned preferred embodiments are described to aid inunderstanding the present invention and variations may be made by oneskilled in the art without departing from the spirit and scope of thepresent invention.

What is claimed is:
 1. A ceramic foam having an inner layer and an outerlayer consisting of: the inner layer of hollow fly ash part produced bya process, in which raw fly ash material containing impurities is coatedwith alumina slurry on the surface thereof, then granulated and then,sintered; and the outer layer of a ceramic shell part produced by saidprocess covering up the surface of the hollow fly ash part.
 2. A ceramicfoam in a form of a hollow granule consisting of: an inner layerconsisting of sintered hollow fly ash containing impurities; and anouter layer consisting of an alumina ceramic shell which covers up asurface of the inner layer.
 3. The ceramic foam according to claim 1,wherein the outer layer of a ceramic shell is highly pure.
 4. Theceramic foam according to claim 2, wherein the outer layer of a ceramicshell is highly pure.